1. Field of the Invention
This invention relates to the oxidation of olefins to .alpha.-.beta.-unsaturated carboxylic acids, and in particular to an improved single step process for the oxidation of propylene to acrylic acid.
Acrylic acid and esters are a highly versatile series of monomers that provide performance characteristics to thousands of polymer formulations. They are flammable, reactive, volatile liquids based on an alpha, beta-unsaturated carboxyl structure. They polymerize readily, yet can be stored and transported safely through the use of very low concentrations of inhibitors. As a consequence of these desirable qualities, they have become an important commodity chemical with a very high growth rate.
2. Description of the Prior Art
Production of acrylic acid and esters has been the subject of intense technological investigation leading to a great variety of processes. The dominant route that has evolved from this work is the catalytic vapour phase oxidation of propylene in two stages, firstly to acrolein vapour and then to acrylic acid vapour using several different catalyst systems followed by aqueous absorption.
Many attempts have been made to simplify the process by combining the oxidation steps. Other investigators have reported on their efforts to include both oxidation steps and the absorption step in a single trickle-bed reactor. Unfortunately, these methods have either produced low yields of acrylic acid based on propylene consumed (low selectivity) or impractical reaction rates.
The oxidation of propylene to acrylic acid in one step employing a palladium metal catalyst supported on carbon black is described in U.S. Pat. No. 3,624,147. However, this process is characterized by yields of 60% or less, based on the amount of propylene converted, operating temperatures generally in excess of 90.degree. C., and elevated pressures. Moreover, substantial amounts of CO.sub.2 are reported as undesired by-products, as well as low reaction rates.
A similar process is reported in J. Catal., 173 (1972) by Seiyama et al., in which palladium black and palladium-activated charcoal were employed for converting propylene to acrylic acid. However, only a stoichiometric, non-catalytic conversion, based on the palladium metal, is taught, thus providing an even less effective method than in the above U.S. Pat.
A different approach using a similar catalyst is reported in EPO published application 145 467 A3. In this process, the palladium catalyst is first activated by preliminary high temperature contact with an olefin, e.g. propylene. Gaseous oxygen and propylene are then passed through the catalyst bed along with a liquid aqueous media which removes the acrylic acid as it is produced, carrying it downwards for recovery. The catalyst is active in promoting the reaction at temperatures as low as 25.degree. C. with high selectivity. Moreover, the undesired by-product, CO.sub.2, is virtually eliminated.
Even higher selectivities are claimed for this process in EPO published application 145 468 where certain surfactants are used in combination with a co-surfactant such as n- or t- butyl alcohol. Similar improvement claims are made in EPO published application 145 469 when a free radical inhibitor such as butylated hydroxytoluene is added to the reactants.
Still another approach to the problem is shown in U.S. Pat. No. 3,792,086 where palladium is combined with phosphoric acid. This catalyst is claimed to constitute a significant advance in the art in that high selectivity is achieved, but the catalyst efficiency deteriorates rapidly due to the formation of a tar that coats the catalyst thus impeding efficient contact with the reactants.
U.S. Pat. No. 3,947,495 claims to have overcome this problem by adding a sulphur modifier. The addition of sulphur (in any one of several forms) apparently inhibits tar formation thus improving both reaction rates and catalyst stability.
U.S. Pat. No. 4,499,307 reveals still another approach to the problem, whereby a solid acid consisting of mixed metal oxides is substituted for phosphoric acid in the catalyst. The catalytically effective metal in this catalyst can be one of a group of noble metals including palladium, gold, and silver, with the preferred composition containing palladium and gold. High selectivities are claimed for this process.
All of the above methods for converting propylene to acrylic acid in a single stage oxidation step suffer from the defect of either low selectivity (i.e. low yield of acrylic acid based on propylene consumed) or low catalyst efficiency (i e low production in terms of grams of acrylic acid produced per gram of palladium per hour) or both.